Hi Steve,
Any time master power is present, whether or not the unit is actually turned on, the internal battery charges. If you bring up the voltage "info item" on the main EFIS screen when the EFIS is on, you can actually watch the "I" (for internal battery) voltage level increase as it charges. Note that from a completely dead state, it can take as much as 10-12 hours to fully charge the battery on master power.